Commonly assigned U.S. Pat. No. 4,326,803, issued Apr. 27, 1982 describes thin film laser gyroscopes, also known as micro-optic gyroscopes (MOGs). Therein, a Sagnac effect is said to define a linear relationship between the rate of rotation of a circular waveguide, or loop, and the difference in frequency in oppositely directed electromagnetic wave disturbances travelling through that waveguide at resonance.
In U.S. Pat. No. 4,326,803 issued Apr. 27, 1982, entitled "Thin Film Laser Gyro" to Lawrence there is described a thin film, passive waveguide that provides a substantially closed circular propagation path for optical signals. A laser and associated beam splitter are adapted to generate two coherent optical signals. Directional couplers introduce the two optical signals to the waveguide in a manner establishing oppositely directed coherent optical signals in the waveguide. In one embodiment, both optical signals are frequency-controlled so that these signals resonate within the waveguide. Frequency control for the optical signals may be achieved by the use of acousto-optic modulators, such as Bragg cells, which shift the frequency of applied optical signals as a function of a frequency of a radio frequency (RF) signal applied to the modulator. With this configuration, the optical signals from a laser and a beam splitter pass along separate paths, through the frequency shifters, directional couplers, and into the waveguide. In addition, optical detectors and associated directional couplers are adapted to detect the intensity of the oppositely-directed optical signals in the waveguide. Servo networks responsive to the detectors generate the RF signals for controlling the frequencies of the optical signals (by way of the modulators) that are injected into the waveguide in their respective directions.
In commonly assigned U.S. Pat. No. 4,674,881, issued Jun. 23, 1987, entitled"Open Loop Thin Film Laser Gyro" to Lawrence et al , there is described a laser gyro that includes a laser and a ring resonator. Here, one servo only allows open-loop performance. Coupling apparatus is provided for coupling counter-propagating beams of light from the laser into the resonator. The frequency of the laser is scanned across the resonance frequency of the resonator in steps superimposed on a direct current level, and the intensity of light in one of the counter-propagating beams is detected during the steps. Apparatus is provided for determining the difference in intensity of this detected light during the steps. Electronics responsive to this difference is used to alter the DC level of the steps to drive the difference to zero. The intensity of light in the other of the counter-propagating beams is then detected during the steps and the difference in intensity is determined for this beam. This difference is indicative of the rotation rate of the gyro.
In commonly assigned U.S. Pat. No. 4,661,964, issued Apr. 28, 1987, entitled "Resonant Waveguide Laser Gyro With A Switched Source" to Haavisto there is described a passive ring resonator laser gyro. The gyro includes a semiconductor laser diode which is switched between two frequencies. The output of the laser diode is coupled into a branching waveguide. The light in this branching waveguide is coupled to a resonant ring waveguide so as to excite counter-propagating beams which are sampled by detectors monitoring the output of waveguides coupled to the ring waveguide. The laser current is switched at a fixed rate between two values corresponding to the resonant frequencies of the two directions of propagation in the waveguide ring. The current values are determined by a resonance tracking servo system. This device functions, electronically, in an open loop manner.
In U.S. Pat. No. 4,274,742, issued Jun. 23, 1981, entitled "Passive Ring Laser Rate of Turn Devices", to Lustig, there is described a ring laser having a passive resonator for the propagation of clockwise and counterclockwise beams of electromagnetic energy. The clockwise and counterclockwise resonant frequencies are oscillated between respective first and second distinct values. A detector is provided for producing signals with amplitudes indicative of the amplitudes of the first and second beams. Circuitry is coupled to the detector means for producing a rotation rate signal proportional to the algebraic combination of the signals indicative of the amplitudes of the first and second beams. By dispensing with closed-loop feedback arrangements to provide an output signal indicative of rotation rate, the ring laser provides very rapid response to changes in the rotation rate of the passive resonator and is thus well adapted for use with high speed data processors. The clockwise and counterclockwise resonant frequencies need not be adjusted precisely to the frequency of either counter-rotating beam.
Accordingly, it is an object of the present invention to provide an improved passive ring resonator laser gyro that utilizes a single frequency shifter and that operates, electronically, in a closed-loop manner.
It is another object of the invention to provide a thin film laser gyroscope that is less complex to manufacture, thereby increasing the device yield and decreasing manufacturing costs.